1. Field of the Invention
This invention relates to the use of shielded optical fibers for telemetering data signals from a plurality of remote sensors to a central receiving station, with particular application to land-based seismic exploration.
2. Discussion of the Prior Art
In land-based seismic exploration a plurality of geophones or geophone groups are laid along a line of survey. Each geophone or geophone group represents a single data channel. There may be several hundred such channels. Customarily, each group is connected to a central recording unit by means of a dedicated pair of wires. The wires are combined into a single multiconductor cable which may be several miles long. As the number of data channels is increased, so also is the size of the cable increased. The ultimate channel capacity of the cable is limited in part by the ability of the seismic crew to physically man-handle the cable when it is to be deployed.
Recently, telemetric systems have come into fashion within the geophysical industry. In known telemetric systems, data are transmitted through data links consisting of a single pair of conductors according to a time-division-multiplex regimen. In a typical system, two or three separate data-input channels are coupled to each of a plurality of data acquisition modules. The analog input data are filtered, amplified, digitized and temporarily stored in the data acquisition units. Upon command from the central recording unit, the digital data words are strobed out to the central recording unit over the telemetric data link. Digital data words from the respective data acquisition units are time-multiplexed. Many of the known systems require a few additional conductors for houskeeping functions such as interrogation, command, power and testing. In some systems, acquired data are summed before transmission.
All of the known telemetric systems have one feature in common: To make a signal connection between a data acquisition unit and the data telemetric system, it is necessary to physically violate the integrity of the data link. That is, the protective insulation must be removed or penetrated to make an electrical connection. In some very long telemetric systems, the data link is cut into a plurality of shorter sections, each of which is terminated by a connector plug.
Wherever physical electrical connections must be made, there is always the chance of broken leads, poor contacts due to moisture or corrosion. The further disadvantage of most known systems is that the location of a data acquisition unit is perforce dictated by the location of the connecting plugs or other cable entry points.